This invention relates to high molecular weight silicone compounds (or silicone polymers) suitable as base resins in resist compositions which lend themselves to fine processing in the manufacture of semiconductor devices or the like; resist compositions which are appropriate when high energy radiation such as deep UV, KrF excimer laser radiation (248 nm), ArF excimer laser radiation (193 nm), electron beam or x-ray is used as a light source for exposure; and a patterning method.
As the LSI technology tends toward higher integration and higher speed, further refinement of pattern rules is required. The light exposure currently employed as versatile technology is now approaching to the essential limit of resolution which is dictated by the wavelength of a light source. It is generally recognized that in light exposure using g-line (436 nm) or i-line (365 nm) as a light source, a pattern rule of about 0.5 xcexcm is the limit. LSIs fabricated by this technique have a degree of integration equivalent to 16 mega-bit DRAM at maximum. At present, LSIs fabricated in the laboratory have reached this stage. It is urgently required to develop a finer patterning technique.
One of approaches for making finer patterns is to use exposure light of a shorter wavelength in forming resist patterns. For the mass production process of 256 mega-bit (working size up to 0.25 xcexcm) DRAM (dynamic random access memory), active research works are now made to substitute a KrF excimer laser of lower wavelength (248 nm) for the i-line (365 nm) as the light source for exposure. However, for the manufacture of DRAM with a degree of integration of 1 G or greater requiring further finer processing technology (working size up to 0.2 xcexcm), a light source of further shorter wavelength is needed. In particular, photography using an ArF excimer laser (193 nm) is now under consideration.
In the case of lithography using light of a short wavelength of 220 nm or shorter as typified by an ArF excimer laser, in order to form fine patterns, photo-resists are required to have new properties which cannot be met by conventional materials. For this reason, since Ito et al. proposed a chemically amplified positive resist composition comprising a polyhydroxystyrene resin whose hydroxyl group is protected with a tert-butoxycarbonyloxy group (t-Boc group), known as PBOCST, and a photoacid generator in the form of an onium salt, a number of high sensitivity, high resolution resist compositions have been developed. Although these resist compositions have high sensitivity and high resolution, formation of a fine pattern having a high aspect ratio is deemed difficult when the mechanical strength of the resultant pattern is taken into account.
A number of resist compositions using polyhydroxystyrene as a base resin and having sensitivity to deep-UV, electron beams and x-rays are known in the art. These resist compositions rely on a single layer resist technique although a two-layer resist technique is advantageous in forming high aspect ratio patterns on stepped substrates. Because of such outstanding problems of substrate steps, light reflection from substrates, and difficult formation of high aspect ratio patterns, the known resist compositions are far from practical use.
It is known that the two-layer resist technique is advantageous in forming high aspect ratio patterns on stepped substrates. It is also known that in order to develop two-layer resist films with conventional alkali developers, silicone polymers having hydrophilic groups such as hydroxyl and carboxyl groups are required. Since silicone polymers having hydroxyl groups directly attached thereto, however, undergo crosslinking reaction in the presence of acid, it is difficult to apply such silanols to chemically amplified positive resist materials.
Recently, as the silicone based positive resist material capable of solving these problems, chemically amplified silicone based positive resist materials comprising polyhydroxybenzylsilsesquioxane, known as a stable alkali soluble silicone polymer, in which some phenolic hydroxyl groups are protected with t-Boc groups, and photoacid generators combined therewith were proposed (Japanese Patent Application Kokai (JP-A) No. 118651/1995 and SPIE, Vol. 1952 (1993), 377).
However, the polymers used in these silicone resist materials have aromatic rings, which cause substantial light absorption at a wavelength of 220 nm or shorter. Thus these prior-art resins as such cannot be applied to photography using light of a short wavelength of 220 nm or shorter. Since the majority of exposure light is absorbed at the surface of resist, exposure light does not penetrate through the resist to the substrate, failing to form a fine resist pattern. (See Sasago et al., xe2x80x9cArF Excimer Laser Lithography (3)xe2x80x94Resist Rating xe2x80x94,xe2x80x9d the preprint of the 35th Applied Physics Society Related Union Meeting, 1P-K4 (1989)).
One exemplary phenyl group-free base resin for silicone based positive resist materials is disclosed in JP-A 323611/1993. Since all hydrophilic groups such as carboxyl and hydroxyl groups necessary to enable alkali development have been protected in this base polymer, many protective groups must be decomposed before exposed areas can be dissolved in the developer. Then the amount of photoacid generator added to this end is increased and the sensitivity is exacerbated. In addition, the decomposition of many protective groups has the high possibility of causing a change of film thickness and generating stresses or bubbles in the film. A resist material having high sensitivity and suited for fine patterning is not available.
An object of the invention is to provide a novel silicone polymer useful as a base resin of a positive resist composition which has a high sensitivity and resolution, is advantageously applicable to the two-layer resist technique suitable for forming high aspect ratio patterns, and can form a heat resistant pattern; a resist composition comprising the polymer as a base resin; and a pattern forming method.
We have found that a high molecular weight silicone compound (often simply referred to as silicone polymer) comprising recurring units of the general formula (1) shown below and having a weight average molecular weight of 1,000 to 50,000, or a high molecular weight silicone compound in which some or all of the hydrogen atoms of carboxyl groups or carboxyl groups and hydroxyl groups in the silicone compound of formula (1) are replaced by acid labile groups of at least one type, as well as a positive resist composition comprising the silicone polymer and a photoacid generator added thereto, or a negative resist composition comprising the silicone polymer, a photoacid generator and a compound crosslinkable under the action of acid, especially resist compositions further comprising a dissolution inhibitor in addition to the photoacid generator, and the resist compositions further comprising a basic compound are effective for increasing the dissolution contrast of resist and especially a dissolution rate after exposure; that the resist compositions further comprising a compound having a group represented by xe2x89xa1Cxe2x80x94COOH in a molecule is effective for improving the PED stability of resist and the edge roughness on nitride film substrates; and that the composition is further improved in ease of coating and storage stability by further blending an acetylene alcohol derivative. Therefore, silicone base resist compositions according to the invention have high transparency, high resolution, improved latitude of exposure, process adaptability, and practical applicability, and are very useful as ultra-LSI resist materials advantageous for precise micro-processing. The present invention is predicated on these findings.
In a first aspect, the invention provides a high molecular weight silicone compound comprising recurring units represented by the following general formula (1) and having a weight average molecular weight of 1,000 to 50,000. Some or all of the hydrogen atoms of carboxyl groups or carboxyl groups and hydroxyl groups in the silicone compound may be replaced by acid labile groups of at least one type. 
Herein, Z is a divalent to hexavalent, non-aromatic, monocyclic or polycyclic hydrocarbon or bridged cyclic hydrocarbon group having 5 to 12 carbon atoms; Z is a divalent to hexavalent, normal or branched hydrocarbon group having 1 to 20 carbon atoms or non-aromatic, monocyclic or polycyclic hydrocarbon or bridged cyclic hydrocarbon group having 3 to 20 carbon atoms, these groups may have a nitrogen, oxygen or sulfur atom interposed in a carbon-to-carbon bond, the hydrogen atom on a carbon atom may be replaced by a halogen, alkoxy, nitro, cyano or acetyl group, and a methylene group in the carbon skeleton may be replaced by a carbonyl group;
letters x, y and z are integers of 1 to 5 corresponding to the valence of Z and Zxe2x80x2;
R1 is a group represented by the following general formula (2a) or (2b); R2 is a normal, branched or cyclic, substituted or unsubstituted, alkyl or alkenyl group having 1 to 8 carbon atoms or a monovalent, non-aromatic, polycyclic hydrocarbon or bridged cyclic hydrocarbon group having 5 to 12 carbon atoms; 
wherein R is hydrogen, hydroxyl group or a normal, branched or cyclic alkyl group having 1 to 20 carbon atoms, Rxe2x80x2 is a normal, branched or cyclic alkylene group having 1 to 20 carbon atoms, these alkyl and alkylene groups may have an oxygen atom interposed in a carbon-to-carbon bond, some of the hydrogen atoms attached to carbon atoms may be replaced by hydroxyl groups; or R and Rxe2x80x2, taken together, may form a ring, and each of R and Rxe2x80x2 is a normal or branched alkylene group of 1 to 8 carbon atoms when they form a ring;
letters p1, p2, p3 and p4 are 0 or positive numbers and satisfy:
1+p2+p3+p4=1,
0 less than p1/(p1+p2+p3+p4)xe2x89xa60.9,
0xe2x89xa6p2/(p1+p2+p3+p4)xe2x89xa60.8,
0xe2x89xa6p3/(p1+p2+p3+p4)xe2x89xa60.7,
0xe2x89xa6p4/(p1+p2+p3+p4)xe2x89xa60.9,
with the proviso that p1 and p4 are not equal to 0 at the same time, and that p3 is not equal to 0 and at least some of R2 groups are monovalent, non-aromatic, polycyclic hydrocarbon or bridged cyclic hydrocarbon groups when p1 is 0.
In one preferred embodiment wherein some of the hydrogen atoms of carboxyl groups or carboxyl groups and hydroxyl groups in said silicone compound are replaced by acid labile groups of at least one type,
more than 0 mol % to 50 mol % of the hydrogen atoms of the carboxyl groups and/or hydroxyl groups are replaced by crosslinking groups having Cxe2x80x94Oxe2x80x94C linkages represented by the following general formula (3a) or (3b) whereby the silicone compound is crosslinked within a molecule and/or between molecules. 
Herein, each of R3 and R4 is hydrogen or a normal, branched or cyclic alkyl group of 1 to 8 carbon atoms, or R3 and R4, taken together, may form a ring, and each of R3 and R4 is a normal or branched alkylene group of 1 to 8 carbon atoms when they form a ring, R5 is a normal, branched or cyclic alkylene group of 1 to 10 carbon atoms, letter d is 0 or an integer of 1 to 10, A is a c-valent aliphatic or alicyclic saturated hydrocarbon group, aromatic hydrocarbon group or heterocyclic group of 1 to 50 carbon atoms, which may have an intervening hetero atom and in which the hydrogen atom attached to a carbon atom may-be partially replaced by a hydroxyl group, carboxyl group, acyl group or fluorine atom, B is xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94NHCOxe2x80x94Oxe2x80x94 or xe2x80x94NHCONHxe2x80x94, letter c is an integer of 2 to 8, and cxe2x80x2 is an integer of 1 to 7.
In a further preferred embodiment, the crosslinking group having Cxe2x80x94Oxe2x80x94C linkages represented by the general formula (3a) or (3b) is represented by the following general formula (3axe2x80x2xe2x80x3) or (3bxe2x80x2xe2x80x3). 
Herein, each of R3 and R4 is hydrogen or a normal, branched or cyclic alkyl group of 1 to 8 carbon atoms, or R3 and R4, taken together, may form a ring, and each of R3 and R4 is a normal or branched alkylene group of 1 to 8 carbon atoms when they form a ring, R5 is a normal, branched or cyclic alkylene group of 1 to 10 carbon atoms, letter d is 0 or an integer of 1 to 5, Axe2x80x2 is a cxe2x80x3-valent normal, branched or cyclic alkylene, alkyltriyl or alkyltetrayl group of 1 to 20 carbon atoms or arylene group of 6 to 30 carbon atoms, which may have an intervening hetero atom and in which the hydrogen atom attached to a carbon atom may be partially replaced by a hydroxyl group, carboxyl group, acyl group or fluorine atom, B is xe2x80x94Coxe2x80x94Oxe2x80x94, xe2x80x94NHCOxe2x80x94Oxe2x80x94 or xe2x80x94NHCONHxe2x80x94, letter cxe2x80x3 is an integer of 2 to 4, and cxe2x80x3xe2x80x2 is an integer of 1 to 3.
In a further preferred embodiment, the acid labile group is at least one group selected from the class consisting of groups of the following general formula (4), groups of the following general formula (5), tertiary alkyl groups of 4 to 20 carbon atoms, trialkylsilyl groups whose alkyl groups each have 1 to 6 carbon atoms, and oxoalkyl groups of 4 to 20 carbon atoms. 
Herein, R6 and R7 each are hydrogen or a normal, branched or cyclic alkyl group of 1 to 18 carbon atoms, R8 is a monovalent hydrocarbon group of 1 to 18 carbon atoms which may have a hetero atom, or R6 and R7, R6 and R8, or R7 and R8, taken together, may form a ring, and R6, R7 and R8 each are a normal or branched alkylene group of 1 to 18 carbon atoms when they form a ring, R9 is a tertiary alkyl group of 4 to 20 carbon atoms, a trialkylsilyl group whose alkyl groups each have 1 to 6 carbon atoms, an oxoalkyl group of 4 to 20 carbon atoms or a group of the above general formula (4), and letter a is an integer of 0 to 6.
In a second aspect, the invention provides a chemically amplified positive resist composition comprising (A) an organic solvent, (B) a base resin in the form of the high molecular weight silicone compound defined above, and (C) a photoacid generator.
In a preferred embodiment, there is provided a positive resist composition comprising (A) an organic solvent, (B) a base resin in the form of the high molecular weight silicone compound comprising recurring units represented by the above-described formula (1), some or all of the hydrogen atoms of carboxyl groups and hydroxyl groups in the silicone compounds being replaced by acid labile groups of at least one type, and (C) a photoacid generator.
The resist composition may further comprise (D) a dissolution inhibitor, (E) a basic compound, (F) a compound having a group represented by xe2x89xa1Cxe2x80x94COOH in a molecule, and/or (G) an acetylene alcohol derivative.
In a preferred embodiment, there is provided a negative resist composition comprising (A) an organic solvent, (B) a base resin in the form of the high molecular weight silicone compound comprising recurring units represented by the above-described formula (1), some of the hydrogen atoms of carboxyl groups or carboxyl groups and hydroxyl groups in the silicone compounds being optionally replaced by acid labile groups of at least one type, (C) a photoacid generator, and (H) a crosslinkable compound by the action of acid.
In a third aspect, the invention provides a method for forming a pattern comprising the steps of (i) applying the resist composition defined above onto a substrate, (ii) heat treating the coated film and then exposing it to actinic radiation having a wavelength of up to 300 nm or electron beams through a photo mask, and (iii) optionally heat treating the exposed film and developing it with a developer.
More particularly, aside from phenyl-bearing silicone polymers as disclosed in JP-A 118651/1994, we have investigated and sought for a polymer in which alkali-soluble groups are not entirely, but partially, protected with acid labile groups, and which provides a high transmittance to light in the deep-UV region and a high resolution. On the other hand, known phenyl-free silicone polymers include a silicone polymer having an ethylcarboxyl group as disclosed in JP-A 323611/1993. When a polysiloxane having hydrogen atoms is reacted with an unsaturated carboxylic acid such as methacrylic acid as in this synthesis process, it is difficult to obtain a silicone polymer as illustrated in JP-A 323611/1993 because of preferential occurrence of addition reaction at the xcex1-position of the unsaturated carboxylic acid. Since effecting hydrosilylation reaction on polymers is difficult in a quantitative sense, it is difficult to provide a constant supply of silicone polymer. Further, the quality control of resist is difficult.
As opposed to these polymers, a resist composition comprising a silicone polymer obtained by introducing acid labile groups into some of the carboxyl groups of an aromatic ring-free high molecular weight silicone compound is highly transmissive to light of a short wavelength of 220 nm or shorter and can form a pattern with such exposure light. The present invention is predicated on this finding.
Further, a resist composition comprising a silicone polymer which has been crosslinked within a molecule and/or between molecules with crosslinking groups having Cxe2x80x94Oxe2x80x94C linkages resulting from reaction of some carboxyl groups therein with an alkenyl ether compound and/or halogenated alkyl ether compound exerts a dissolution inhibitory effect even in a low proportion of crosslinking and exhibits improved heat resistance on account of the increased molecular weight due to crosslinking. Additionally, since the polymer reduces its molecular weight as a result of deblocking of crosslinking groups after exposure rather than before exposure, the resist film can be increased in dissolution contrast and eventually has a high sensitivity and high resolution. Also, since the problems of footing and a difficultly soluble subsurface stratum are minimized, the resist composition allows the size and configuration of a resist pattern to be controlled as desired in terms of composition, and offers improved process adaptability. The present invention is also predicated on this finding.